Upgrade to ETT 3.x.x

[elichad]

We're on version 2.4.0 currently. There are a few new features and bug fixes in 3.x.x already, including ones that affect the RO-Crate site.

Sub-tasks after an initial look locally:

• "Related pages" dropdown no longer works on domain pages
• Cards are highlighted in a dark colour on focus which affects contrast badly - tweak the colour or revert this to previous behaviour
• Links in cards should be colour $dark not blue (as the whole card is the link)

[elichad]

The problems listed all derive from our override of section-navigation-tiles.html as the styling was significantly changed in ETT v3. We do still need to override it but we should be able to bring the file in line with v3.

[elichad]

Deprioritising as #336 is fixed. The work I started on this is in https://github.com/elichad/ro-crate/tree/upgrade-ett